The invention relates to digital controllers that are used to control industrial machines and processes.
A programmable controller typically has a main processor that is connected through one or more I/O interface modules to I/O devices that control a machine or process. In early controllers the main processors and the I/O modules were housed separately, the I/O modules being placed in an I/O rack that was located near the machine or process being controlled. Several of these I/O racks could be placed at different locations and connected through cables to the main processor, however, the length of the cables between the I/O modules and the main processor was limited, because such cables were used for parallel data transmission.
Another type of programmable controller has provided a processor and I/O racks that could communicate over serial transmission lines, to permit the I/O racks to be located more remotely, at destinations up to 10,000 feet away from the main processor. Such a controller is described in Galdun et al, U.S. Pat. No. 4,254,473 issued Mar. 3, 1981, entitled "Rack Adapter for Serially Connected I/O Interface Racks," and assigned to the assignee of the present invention. This system employes a scanner circuit of an earlier period with a processor-based adapted circuit that allows serial connection of I/O racks along serial data channels. The scanner and adapted transmit words of conventional I/O status data in a synchronized, bit-oriented fashion.
The ability to interface I/O devices with a programmable controller has been recently enhanced with the invention of intelligent high density I/O modules of the type disclosed in Struger et al, U.S. Pat. No. 4,293,924, issued Oct. 6, 1981, and assigned to the assignee of the present invention. Such high density I/O modules provide the I/O capacity to up to eight conventional I/O modules and are well-suited for interfacing analog I/O devices to the main processor. Because the scanner and adapter shown in Galdun et al, U.S. Pat. No. 4,254,473 are not operable with such high density modules, block transfer of data to and from high density modules has been limited to single rack systems where the main processor is located in the same rack as the high density module, or to local systems, where the main processor is located within 50 feet of its I/O interface rack. Block transfer has also been limited to a single I/O rack per channel in local systems that have multiple communication channels connected in parallel.